1. Origin of the Invention.
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. .sctn.202) in which the Contractor has elected not to retain title.
2. Field of the Invention.
The present invention relates to apparatus and methods for detecting infrared radiation. Additionally, the present invention relates to apparatus and methods for generating infrared radiation.
3. Description of the Prior Art and Related Information
The detection of infrared radiation in the long wavelength region, for example, beyond about 15 microns, is important for a variety of applications including astronomy, earth sensing from space, etc. The detection of infrared radiation in this long wavelength region, however, presents considerable difficulties for presently known detection approaches. Such presently known approaches generally employ one of three categories of detectors: (1) extrinsic photoconducting semiconductors; (2) bolometers; and (3) intrinsic photoconducting semiconductors. All three types of detectors have significant shortcomings when applied to the long wavelength infrared radiation regime.
Extrinsic photoconducting semiconductors employ semiconductors doped with very small amounts of impurities to provide infrared detection through photoconduction. The impurities act to change the conduction properties of the semiconductor when it absorbs infrared radiation. These extrinsic infrared detectors are very sensitive to the amount of dopant and to the processing of the doped semiconductor material. As a result, it is difficult to repeatedly control the detection characteristics of such extrinsic infrared detectors. Also, extrinsic infrared detectors suffer from very low efficiency of detection and susceptibility to noise. Although the noise may be reduced by means of cryogenic cooling to very low temperatures, for example 1.degree. K., the weight, size, cost and logistic penalties of such cryogenics introduce significant practical problems.
Bolometer infrared detectors employ the conversion of the incident infrared energy to heat. The heat is in turn measured by means of a temperature rise in the bolometer. Undoped silicon is frequently employed as a bolometer material. Bolometers have high noise at elevated temperatures Bolometers, therefore, require cooling to extremely low temperatures to provide suitable sensitivity and signal to noise ratios; for example, cooling to as low as 0.1.degree. K. is required for applications requiring good sensitivity. It will be appreciated that cooling to this very low temperature requires extremely sophisticated and expensive cryogenics. Also, bolometers employed as infrared detectors are unable to discriminate between wavelengths since any incident radiation will result in some absorption and some heating of the material. Accordingly, bolometers are not suitable for applications requiring the detector to provide a wavelength discrimination or photospectrometry function.
The most desirable infrared detectors for most applications are intrinsic semiconductor devices which employ absorption of photons across the forbidden energy gap of the semiconductor to create electron/hole pairs which carriers in turn create a detectable current or potential. While these intrinsic semiconductor devices are effective at shorter wavelength regimes, for longer wavelengths, beyond 8 microns, for example, no easily prepared semiconductor materials are available. By carefully tailoring alloys of semiconductors, it has been possible to obtain materials suitable for intrinsic photodetection in the 8-14 micron regime. For example, the alloys Hg.sub.1-x Cd.sub.x Te or Pb.sub.x Sn.sub.1-x Te, where the energy gap is carefully controlled by control of x, have been employed. For achieving intrinsic semiconductor detectors for the infrared region beyond wavelengths of about 14 microns, however, the control necessary in the composition of such alloy semiconductors is too demanding to make such detectors a practical solution.
Accordingly, a need presently exists for an efficient infrared detection device for wavelengths in the infrared region beyond approximately 14 microns. Additionally, a need presently exists for a long wavelength infrared detector having good signal to noise ratios at elevated temperatures. A need further exists for an infrared detector having wavelength discrimination capabilities.